1) Field of the Invention
The present invention relates to an optical transceiver, a connector, a substrate unit, an optical transmitter, an optical receiver, and a semiconductor device, that perform signal transmission via interface pins.
2) Description of the Related Art
The optical transceiver performs transmission and reception as follows. That is, when transmitting, the optical transceiver converts the data signal, which is an electric signal, into an optical signal, and transmits the optical signal via an optical fiber for transmission. On the other hand, when receiving, the optical transceiver receives an optical signal via an optical fiber for reception, and converts the optical signal into the data signal, which is an electric signal.
The optical transceiver is sometimes fitted to a substrate of a host system device (also referred to as an optical transmission device), which is externally equipped. The host system device generates digital data signals, distributes and transmits the generated digital data signals to a plurality of optical transmitters and receivers, and carries out various processing with respect to the data signal transmitted from the transmitters and receivers.
When fitting the optical transceiver to the substrate of the host system device, an electrical interface is obtained by connecting the optical transceiver to a system substrate of the host system device, using a plurality of interface pins protruding downwards from the bottom of a housing. In other words, no connector is not used when fitting the optical transceiver to the substrate. The interface pins are used for transferring power supply, clock and signals, such as power-supply voltage, control signals, high-speed pulse signals, and high-speed clock signals.
In the optical communication system that transmits optical signals via the optical fiber, the transmission rate of the optical signal has recently been increased rapidly, in order to respond to an increase in the communication traffic accompanying popularization of the Internet. In the optical transmitter/receiver, the transmission rate is now shifting from 2.5 Gb/s to 10 Gb/s, and research and development is now under way to realize the transmission rate of 40 Gb/s. Accompanying this, the optical transceiver is also required to have the capability to handle signal with high transmission rate.
FIGS. 27A and 27B show a schematic configuration of a conventional optical transceiver and an interface structure between the system substrate and the optical transceiver. FIG. 27A is a top view with an upper lid being removed, and FIG. 27B is a sectional side view taken along the line A-A shown in FIG. 27A. FIGS. 27A and 27B show an example of an optical transceiver 117 that performs signal transmission at a data rate of 2.5 Gb/s.
A housing 101 of the optical transceiver 117 has a rectangular shape and it is formed by bending a metal plate on four sides. The metal plate has a thickness of from about 0.5 to 1 mm. The housing 101 has four sidewalls 101b, a bottom 101a, and an upper lid 116. Long holes 101c for interface are provided near two sides on the bottom 101a. At least three protrusions 101d are provided (see FIG. 27B) on the outer surface of the bottom 101a. These protrusions 101d abut on the system substrate 114, so that the housing 101 is stably seated on the system substrate 114.
A substrate 109 is provided in the housing 101. A laser diode driver 102, a laser diode module 103 having a laser diode (LD) provided therein, a photo diode module 104 having a photo diode (PD) provided therein, a receiving circuit 105 including a PLL (phase-locked loop) circuit and a data identification and generation circuit, and a control circuit 108 including a power supply circuit and various control devices are mounted on the substrate 109. An optical fiber 106 for transmission is connected to the laser diode module 103, and an optical fiber 107 for reception is connected to the photo diode module 104. Substrate holding members 115 provided on the bottom 101a support the substrate 109.
Substrate lines 110a (transmission side), 110b (transmission side), 110c (reception side), 110d (reception side), 111a and 111b are formed on the substrate 109. The substrate lines 111a connect low speed interface pins to the control circuit 108. The substrate lines 111b connect the laser diode driver 102, the laser diode module 103, the photo diode module 104, and the receiving circuit 105 to the control circuit 108, respectively. The substrate lines 110a (transmission side), 110b (transmission side), 110c (reception side) and 110d (reception side) have such a structure that they can transmit data signals and clock signals, which may even be high frequency signals. Further, about four to ten high speed interface pins (white circles) 112a (transmission side), 112c (reception side), which can transmit data signals and clock signals, which may even be high frequency signals, are provided. Moreover, the low speed interface pins 112b, to which low frequency control signals and dc voltage are supplied, and ground pins (black circles) 113 for grounding are connected and fixed to the substrate 109. The substrate 109 is electrically connected to the system substrate 114, via the interface pins 112a to 112c (collectively referred to as interface pins 112) and the ground pins 113. Thereby, an exchange of signals can be performed between the optical transceiver 117 and the host system device.
The interface pins 112 and the ground pins 113 are linearly arranged in a row, at each edge on the long side of the substrate 109, and protrude downwards from the long holes 101c formed in the housing 101. The respective pins 112 and 113 are inserted into pin holes provided in the system substrate 114, and soldered. Thus, an electric connection between the substrate 109 and the system substrate 114 can be realized without the use of a connector.
The substrate holding member 115 is arranged at four corners on the bottom 101a of the housing 101. Each of the substrate holding members 115 is joined with the substrate 109 atone end, and fixed on the bottom 101a of the housing 101 at the other end. Therefore, there is a gap of several millimeters between the substrate 109 and the bottom 101a of the housing 101. These substrate holding members 115 are provided so that the pattern wiring on the back of the substrate 109 does not come in contact with the bottom 101a of the metal housing 101.
The conventional optical transceiver operates as described below. A data signal (pulse signal) of 2.5 Gb/s and a clock signal of 2.5 GHz are input from the system substrate 114 through the interface pins 112a, and these signals are transmitted to the laser diode driver 102 via the substrate lines 110b. Further, power supply voltage and control signals are supplied to the control circuit 108 through the interface pins 112b and the substrate lines 11a. 
The laser diode driver 102 generates a modulation signal (pulse signal) Im for driving the laser diode module 103, based on the data signal of 2.5 Gb/s and the clock signal of 2.5 GHz. The modulation signal Im generated in the laser diode driver 102 is transmitted to the laser diode module 103 via the substrate line 110a. The control circuit 108 supplies dc voltage to the laser diode driver 102, the laser diode module 103, the photo diode module 104 and the receiving circuit 105 via the substrate lines 111b, and monitors the respective equipment. As a result, the laser diode in the laser diode module 103 emits light, and an optical signal is emitted, with the intensity thereof modulated. The emitted optical signal enters into the end face of the optical fiber 106, and hence an optical signal Po is propagated in the optical fiber 106.
The photo diode module 104 receives an optical signal Pi via the optical fiber 107, and photoelectrically exchanges the optical signal to a current signal by the built-in photo diode, then converts the current signal to a voltage signal by a built-in preamplifier, and transmits the converted voltage signal to the receiving circuit 105 via the substrate line 110c. The receiving circuit 105 extracts a clock based on the voltage signal transmitted from the photo diode module 104, and regenerates the data signal. The data signal and the clock signal regenerated by the receiving circuit 105 are input from the substrate line 110d to one end of the interface pin 112c, and are impedance-converted. The signal input to the one end of the interface pin 112c is input to the system substrate 114 connected to the other end thereof through the interface pin 112c. 
As a result of an increase in the transmission rate of the optical signal, problems that are described below arise in the conventional optical transceiver.
For example, when a data signal of 10 Gb/s higher in rate than 2.5 Gb/s and a clock signal of 10 GHz higher in rate than 2.5 GHz are to be transferred between the optical transceiver and the system substrate 114, the transmission characteristic deteriorates, causing a problem in that high speed signals cannot be transmitted, unless the characteristic impedance (normally 50Ω) of the substrate lines 110b is matched with that of the interface pins 112a, or the characteristic impedance (normally 50Ω) of substrate lines 110d is matched with that of the interface pins 112c. 
In the conventional optical transceiver, there is a gap 118 of about 2 to 5 mm between the substrate 109 and the system substrate 114, depending on the height of the substrate holding member 115, the thickness of the bottom plate 101a of the housing 101, and the height of the protrusion 101d provided in the housing 101.
Due to the existence of this gap 118, an air layer exists between the substrate 109 and the system substrate 114, and it is actually difficult to match the characteristic impedance of the interface pins with the characteristic impedance (normally 50Ω) of the substrate lines 110b and 110d. 
Therefore, in the conventional optical transceiver, there is a problem in that when a high speed data signal of about 10 Gb/s or higher and a high speed clock signal of about 10 GHz of higher are to be transmitted, the transmission characteristic deteriorates. The deterioration in the transmission characteristic is allowable in putting the optical transceiver to practical use, for a data signal having a bit rate of about 2.5 Gb/s, but if the bit rate becomes about 10 Gb/s, deterioration in the transmission characteristic becomes noticeable, and cannot be ignored.
If an interface connector (coaxial connector) with the characteristic impedance being controlled, or a connection line using a waveguide is used instead of the interface pin, impedance characteristic that is electrically preferable can be obtained, but the quantity of connection lines required for transmission of the data signal and the clock signal is about four to ten. Use of such connection lines increases the size of the interface apparatus and increases the cost, and hence it is not practical for the optical transceiver that is required to be cheap and installed in a large quantity.
Thus, there is a problem in the conventional optical transceiver, that when a data signal of about 10 Gb/s and a clock signal of about 10 GHz are transmitted using interface pins, it is difficult to match the characteristic impedance of the substrate line with that of the interface pins.